In many types of well applications, fluid samples are captured downhole and tested to help evaluate well fluid and/or geologic formation parameters. To obtain the desired fluid sample, a sampling tool is deployed downhole into a wellbore, and the fluid sample is drawn through a port and into a sampling chamber in the tool. A variety of pistons and/or other devices may be used to draw the fluid sample into the sample chamber. Once the fluid sample is in the chamber, the chamber is closed prior to pulling the sampling tool back to the surface, as accurate compositional and PVT analyses of the fluid sample may depend upon the fluid sample remaining at downhole conditions. Problems sometimes occur, however, due to inadvertent, premature closing and/or opening of the sample chamber.